Fluid catalytic cracking process

ABSTRACT

A FLUIDIZED CATALYTIC CRACKING PROCESS EMPLOYING MULTIPLE ELONGATED RISER REACTION ZONES WHEREIN A CHARGE STOCK, COMPRISING A LIGHT FRACTION BOILING IN THE 400-650* F. RANGE AND HAVING A POUR POINT OF +10* F. OR HIGHER AND A HEAVY FRACTION BOILING ABOVE 650* F., IS CONVERTED TO PRODUCE SUBSTANTIAL YIELDS OF HIGH OCTANE DEBUTANIZED NAPHTHA AND LIGHT CYCLE GAS OIL BOILING IN 400-650* F. RANGE AND HAVING A POUR POINT OF 0* F. OR LESS. THE LIGHT FRACTION IS SEPARATELY CRACKED IN A SECOND RISER REACTION ZONE ZONE AT HIGH TEMPERATURE AND HIGH CONVERSION FOR CONTROL OF LIGHT CYCLE GAS OIL PRODUCT POUR POINT. THE HEAVY FRACTION IS SEPARATELY CRACKED IN A SECOND RISER REACTION ZONE AT MODERATE TEMPERATURE AND MODERATE CONVERSION FOR CONTROL OF NAPHTHA TO LIGHT CYCLE GAS OIL PRODUCT RATIO.

United States Patent Oifice 3,799,864 Patented Mar. 26, 1974 3,799,864 FLUID CATALYTIC CRACKING PROCESS Dorrance P. Bunn, In, Houston, and Roy E. Pratt, Groves, Tex., assiguors to Texaco Inc., New York, N.Y. N Drawing. Filed Jan. 2, 1973, Ser. No. 320,036 Int. Cl. B01j 9/20; C10g 11/00 US. Cl. 208-80 9 Claims ABSTRACT OF THE DISCLOSURE A fluidized catalytic cracking process employing multiple elongated riser reaction zones wherein a charge stock, comprising a light fraction boiling in the 400-650 F. range and having a pour point of +10 F. or higher and a heavy fraction boiling above 650 F., is converted to produce substantial yields of high octane debutamzed naphtha and light cycle gas oil boiling in 400-650 F. range and having a pour point of 0' F. or less. The light fraction is separately cracked in a first riser reaction zone at high temperature and high conversion for control of light cycle gas oil product pour point. The heavy fraction is separately cracked in a second riser react1on zone at moderate temperature and moderate conversion for control of naphtha to light cycle gas oil product ratio.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to fluidized catalytic cracking of relatively heavy petroleum fractions into useful products including naphtha suitable for use as motor gasoline and light cycle gas oil suitable for use in furnace oil. More particularly, the present invention relates to an improved fluidized catalytic cracking process for conversion of highly paraflinic or waxy petroleum fractions into products having commercially desirable properties.

Description of the prior art Fluidized catalytic cracking processes for conversion of petroleum fractions are well known in the prior art. Such cracking processes are commercially employed for conversion of relatively heavy hydrocarbons such as atmospheric gas oils, vacuum gas oils, topped crudes, residuum, etc., into commercially more valuable products such as olefins and isoparaflins for alkylation process charge stock, naphthas for gasoline blending components, light cycle gas oils for furnace oils, etc. Such fluidized catalytic cracking processes generally comprise process steps including contacting a hot, regenerated catalyst with a hydrocarbon feed in a reaction zone under cracking conditions; separating cracked hydrocarbon vapors from used cracking catalyst; stripping volatile hydrocarbons from used cracking catalyst with a stripping vapor; regenerating stripped catalyst by burning carbonaceous deposits, e.g., coke, therefrom with a molecular oxygen containing gas; returning regenerated catalyst to said reaction zone for reaction with additional amounts of hydrocarbon feed; and separating cracked hydrocarbon vapors into fractions including a gas product, a naphtha product, a light cycle oil product and one or more heavier fractions boiling above the light cycle oil boiling range.

Furnace oils, which are also known as fuel oil No. 1, fuel oil N0. 2, and heating oil, are particularly useful for home and industrial heating purposes. The end use of these oils requires that they have a pour point of 0 F., or lower. A conventional source of furnace oil is from the efiluent stream from a catalytic cracker. In particular, light cycle gas oils often serve as blending stock for these furnace oils. Fuel oil No. 2 and heating oil, which come within the broad definition of furnace oil, have a pour point requirement of 0 F., and a boiling range of about 430-650" F. Another furnace oil is fuel oil No. 1 which typically can have a 430-550" F. boiling range with a pour point of about 30 F. Although light cycle gas oils meet most specifications for furnace oils, they usually must be subjected to some further processing to provide a saleable product. Typically, caustic washing is required to complete the processing.

With increasing emphasis upon reduction of air pollution from combustion of furnace oils, demand for low sulfur content furnace oils increases. Low sulfur content crude oils may be processed for production of low sulfur content furnace oils. Although low-sulfur crudes are highly desirable sources of gas oils because of their sulfur content, they are often highly paraifinic which results in a substantially higher pour point for gas oils obtained therefrom than from crude oils employed heretofore. Light cycle gas oils obtained from catalytic cracking of these waxy gas oils also have high pour points which necessitate additional processing or substantial modifications to present processing to obtain the furnace oil pour point specification of 0 F. and below. Although dewaxing processes well known in the art may be an obvious solution to the problem and will reduce the pour point of the light cycle gas oils, the utilization of solvent de- Waxing, urea dewaxing or other dewaxing processes increases significantly the cost of producing furnace oils.

In recent years improved cracking catalysts, including zeolitic molecular sieves and high alumina catalysts, have been developed. Such improved catalysts have higher conversion activity wherein components of a hydrocarbon charge stock boiling higher than about 430 F. are converted into hydrocarbons boiling below 430 F. and coke. Additionally, such improved catalysts have increased naphtha selectivity wherein a greater portion of the hydrocarbons converted appear as naphtha. When such improved catalysts are employed in a fluidized catalytic cracking process, it has been found preferable to contact a hydrocarbon fraction for a relatively short residence time with such improved catalysts at cracking conditions. Such contact has been found most effective when the catalyst is dispersed in a vapor stream of the hydrocarbon charge stock which is moving with suflicient velocity to maintain the catalyst entrained as a dilute suspension with a minimum of back mixing along the path of flow. Such contact between catalyst and hydrocarbon vapors may be effectively obtained employing elongated reaction conduits having a substantial vertical component, such as are shown in US. Pat. 3,448,037, issued June 3, 1969. One or more such elongated reaction conduits may be employed in a fluidized catalytic cracking unit.

In a fluidized catalyzed cracking process wherein elongated reaction conduits are employed, reaction conditions for the conversion of relatively heavy petroleum fractions into desired cracked products include reaction conduitoutlet temperatures in the range of 850-1200 F., preferably 925-1000 F. or higher. Reaction zone pressures of from about 5-50 p.s.i. g., catalyst oil weight ratios of about 2-20 lbs. of catalyst per pound of oil, vapor residence times in said reaction conduits of about 05-10 seconds and preferably 1-5 seconds, superficial vapor velocities near reaction conduit entrances of about 10- 25 feet per second and velocities near the outlets of about 20-60 feet per second. Regeneration of used cracking catalyst, wherein carbonaceous deposits are burned from the catalyst with molecular oxygen, may be carried out at temperatures in the range of 1100-1500" F. and at regeneration vessel dilute phase pressures in the range of 5-50 p.s.i.g., preferably 20-40 p.s.i.g. Combinations of the above reaction conditions may be employed to obtain conversions of the hydrocarbon charge in the range of -95 percent and preferably -90 percent. It is also known, under certain conditions, to recycle part or all of the unconverted hydrocarbons recovered from the reaction zone effluent to the reaction zone for additional conversion.

Such fluidized catalytic cracking processes are effective for converting a variety of petroleum fractions and other hydrocarbon oils into desirable products including C -C olefins and isoparaflins, naphtha boiling range hydrocarbons having relatively high octane values, and light cycle oil falling in the boiling range of about 400 to 650 F. suitable for use as furnace oil blend stock. However, when petroleum fractions having substantial amounts of long chain, high molecular weight paraflinic hydrocarbons (commonly known as wax) are employed as feed to a fluidized catalytic cracking process, product quality of the light cycle oil and heavier product fractions is impaired. Such paraflinic petroleum fractions are characterized by having a high pour point temperature which makes them unattractive for use as furnace oil blend stock as the oils tend to become solids at relatively high temperatures thus making them difficult to transport and store in cold climates where furnace oils are commonly used. Light cycle oil products from a fluid cracking process wherein paraffinic petroleum fractions are charged also tend to have pour point temperatures substantially above standard accepted values, about F. or less, of furnace oils for which therse light cycle oils are otherwise Well suited. Pour point temperatures for light cycle oil products of parafiinic petroleum fractions may be lowered by increasing the severity of the fluid cracking reaction. However, increasing or decreasing conversion severity of a 650 F.+ gas oil above or below about the 60-70% range decreases the yield of light cycle gas oil product.

SUMMARY OF THE INVENTION Now, according to the method of the present invention, we have discovered a fluidized catalytic cracking process for converting paraflinic hydrocarbon fractions to produce good yields of high octane naphthas and light cycle gas oils having low pour point temperatures. Such improved process comprises separating a paraifinic hydrocarbon fraction having a pour point of greater than 0 F. into a light fraction boiling above about 400 F. and below about 650 F. and a heavy fraction boiling above about 650 F. The light fraction is cracked in a first dilute phase riser cracking zone in the presence of a cracking catalyst under severe conditions to convert at least about 75% of such light fraction into C -C hydrocarbons, naphtha, and coke. The heavy fraction is cracked in a second dilute phase riser cracking zone under less severe conditions for cracking of a substantial portion of such heavy fractions into hydrocarbons boiling below 650 F. and including the production of a substantial amount of naphtha and of light cycle gas oil boiling in the 400650 F. range.

Eflluent streams from the first and second dilute phase reaction zones are combined in a common disengaging vessel for separation of cracked hydrocarbons from used catalyst. Cracked hydrocarbons from the disengaging vessel are separated in a product fractionation zone into at least a gas fraction, a naphtha fraction, a light cycle gas oil fraction, and one or more fractions higher boiling than light cycle gas oil. Reaction conditions in the first dilute phase cracking zone are controlled to maintain the pour point temperature of the light cycle gas oil from the product fractionation zone at a selected value less than +l0 F. Reaction conditions in the second dilute phase reaction zone are controlled to produce the desired product distribution from the product fractionation zone.

One advantage of the present invention is the pour point temperature of the product light cycle gas oil may be maintained within an acceptable commercial range for use as a furnace oil blending stock and simultaneously the product ratio of naphtha to light cycle gas oil may be varied over a wide range of values. This and other advantages will be described in the detailed description of the invention which follows.

Another method for obtaining furnace oils having 0 F. pour point from high pour point light cycle gas oil comprises fractionating such light cycle gas oils into a light fraction having an ASTM distillation end point temperature of less than 650 F. and a heavier fraction. The light fraction end point temperature is adjusted to maintain the pour point temperature below 0 F., and such light fraction is employed as furnace oil. This fractionation process, however, substantially decreases the amount of furnace oil which may be obtained from a high pour point light cycle gas oil.

DETAILED DESCRIPTION OF THE INVENTION We have surprisingly discovered that charge stocks containing substantial amounts of waxy paraflin hydrocarbons and comprising a light fraction boiling within the 400-650 F. range and a heavy fraction boiling above about 650 F., may be catalytically cracked for production of high octane naphtha and low pour point temperature light cycle gas oil. The naphtha product is useful as a gasoline blend stock and the low pour point temperature light cycle gas oil is useful as furnace oil blend stock. By following the method of the present invention we have also found that the product ratio of naphtha to light cycle gas oil may be varied over a wide range without adversely affecting product quality.

Charge stocks within the contemplation of the present invention include those petroleum fractions and hydrocarbon streams containing substantial amounts of long straight chain waxy paraflin hydrocarbons and comprising a high pour point temperature light fraction boiling within the 400-650 F. range and a heavy fraction boiling above about 650 Examples of such charge stocks include Waxy crudes. topped crudes, gas oils, etc. as well as other highly paraffinic hydrocarbon streams. The waxy paraflin content of such charge stocks imparts high pour point temperatures thereto such that the light fractions of such charge stocks, although boiling Within the furnace oil :boiling range, have unacceptable high pour point temperatures, e.g., above +10 'F., for use as furnace oil blend stocks. Heavy fractions of such charge stocks, boiling above about 650 F. are generally suitable charge for conventional fluid catalytic cracking processes wherein high yields of naphtha and relatively low pour point light cycle gas oil may be obtained at moderate reaction severities. However, the high pour point light fraction of such a waxy charge stock is of a character such that when charged to a conventional fluid catalytic cracking process, the pour point temperature of produced light cycle gas oil increases substantially. Such high pour point light cycle gas oil is less useful or unacceptable as a furnace oil blend stock.

In the process of the present invention, the steps include:

(a) Separating a waxy charge stock into a light fraction boiling up to about 650 F. and having a pour point temperature above about -+10 F., and a heavy fraction boiling above about 650 F.;

(.b) Contacting, in a first elongated riser reaction zone, light fraction and hot regenerated cracking catalyst at a first riser outlet temperature in the range of about 950- 1100 F. for conversion of at least about of said light fraction;

(c) Contacting, in a second elongated riser reaction zone. heavy fraction and hot regenerated cracking catalyst at a second riser outlet temperature in the range of about 900 to 1050 F. for conversion of from about 60- of said heavy fraction;

(d) Discharging efiluent from said first and second risers, comprising vaporous reaction products and used catalyst, into a disengaging space above a dense phase fluidized catalyst bed;

(e) Collecting used catalyst from step (d) in said fluidized bed, and withdrawing, stripping, regenerating, and introducing such catalyst into steps. (b) and (c) above;

(f) Fractionating vaporous products from said disengaging space in a product fractionation zone, into at least a C and lighter fraction, a C 430" F. naphtha fraction, a light cycle gas oil fraction, and one or more fractions higher boiling than light cycle gas oil.

We have discovered that in such a fluid catalytic cracking process, the pour point of the light cycle gas oil components produced from cracking the light fraction of the waxy charge stock is substantially affected by the temperature employed and degree of conversion obtained in the cracking reaction. That is, the pour point temperature decreases with increased conversion and increased reaction temperature. Additionally the octane of naphtha produced from cracking the light fraction of the waxy charge stock increases with increased reaction temperatures. Consequently, by cracking the light fraction of the waxy charge stock at a high temperature, above about 950 F., to a high degree of conversion, at least about 80%, the light fraction may be converted into substantial yields of high octane naphtha and relatively low pour point temperature light cycle oil.

We have observed that the pour point temperature of a light cycle gas oil, obtained from cracking the heavy fraction of a waxy charge stock, is substantially not dependent upon the severity or temperature of the cracking reaction. Thus, the heavy fraction may be cracked over a wide range of conversions and the pour point of the light cycle gas oil component is substantially unaffected while the volume of light cycle gas oil varies substantiall Aiz cording to the present invention, the light fraction of the waxy charge stock is cracked to a high conversion at a high temperature for production of a substantial amount of high octane naphtha and a small amount of light cycle gas oil component and the heavy fraction of the waxy charge stock is cracked to moderate conversion for production of substantial amounts of both naphtha and low pour point light cycle gas oil.

Cracked vapors from both the light fraction cracking step and the heavy fraction cracking step are recovered in admixture with one another for fractionation into the desired products. Consequently, the naphtha product comprises naphtha obtained from cracking both light and heavy fractions of the waxy charge stock and the llght cycle gas oil product comprises light cycle gas oil components from both the light and cracked heavy fractions of the waxy charge stock. The light cycle gas oil product obtained from the process of this invention is one suitable for use as furnace oil blend stock, has a boiling range of from about 400 to about 650 F. and a pour point temperature of less than F., preferably less than about 0 F. Pour point of the product light cycle gas oil is controlled, according to the present invention, by adjusting conversion and reaction temperature of the light fraction cracking step to obtain the desired pour point temperature for the product light cycle gas oil. The ratio of product naphtha to product light cycle gas oil is controlled by adjusting the conversion of the heavy fraction of the waxy crude charge stock.

A fluidized catalytic cracking unit configuration useful for practicing the method of the present invention may be one such as is shown in US. Pat. 3,448,037, patented June 3, 1969. In addition, other process configurations employing a plurality of riser reaction zones, may also be used within the contemplation of the present invention.

Catalysts contemplated for use in the present invention are those fluid catalytic cracking catalysts having high activity for conversion of hydrocarbons boiling above about 430 F. into hydrocarbons boiling below about 430 F. and coke. Also, it is contemplated that such catalysts will have high selectivity for production of naptha boiling range hydrocarbons from that portion of the hydrocarbon which are converted. For example, zeolite catalysts, high alumina catalyst, and other catalysts having the desired high conversion activity and naphtha selectivity may be employed within the con- 6 templation of the present invention. As a more specific example, the zeolitic catalyst, as described in referenced patent US. 3,448,037, are particularly useful as catalysts for the practice of the present invention. Such composite crystalline zeolite catalysts comprise about 1 to 25 weight percent zeolite, 10 to 50 weight percent alumina, and the remainder silica. In general the zeolitic catalysts, which form the high activity components of the catalyst, are alkaline metal, crystalline alumino-silicate which have been treated to replace all or at least a substantial portion of the original alkaline metal ions with other cations such as hydrogen and/or a metal or combination of metals, such as barium, calcium, magnesium, manganese, or rare earth metals such as cerium, lanthanum, neodymium, praseodymium, samarium, and yttrium.

As contemplated herein, a waxy charge stock is separated into a light fraction and a heavy fraction. The fractions are converted in the presence of high activity cracking catalysts through separate elongated riser reaction zones. With regard to the light fraction, comprising hydrocarbons boiling in the range of about 400 to about 650 F. and having a pour point of about +10 F. or higher, conversion is undertaken at a temperature ranging from about 950 F. to about 1100 F. and preferably at a temperature ranging from 1000 to 1100 F., wherein the light fraction undergoes conversion above about and preferably from about to Conversion of the light fraction of the waxy charge stock within the range stated above is accomplished by operating within the following parameters: feed preheat temperature in the range of 350-800 F., catalyst to oil weight ratio within the range of about 2:1 to 20:1, residence time of hydrocarbon within the riser of from about 1 to 10 seconds and preferably from 1 to 5 seconds, superficial vapor velocities of from about 10 to 60 feet per second, and reaction zone pressures in the range of about 5 to 50 p.s.i.g. Such reaction conditions may be varied within the ranges given to obtain conversion and reaction temperatures sufficient to yield a light cycle gas oil product from the process of the present invention boiling in the range of from about 400 to about 650 F. and having a pour point temperature of +l0 F. or lower, preferably 0 F. or lower.

With regard to cracking of the heavy fraction of the waxy charge stock, conversion is undertaken in a second elongated riser reaction zone at a temperature of from about 900 to 1050 F. within the heavy fraction undergoes a conversion within the range of about 60 to 85%, preferably; in the range of about 65 to 75%. Conversion of the heavy fraction with the range stated above is accomplished by operating as follows: heavy fraction preheat temperature in the range of about 450 to 750 F., catalyst to oil weight ratios in the range of 2:1 to 20:1, superficial vapor velocities in the second riser of from about 10 to 60 feet per second, reaction zone pressure of about 5 to 50 p.s.i.g., residence time of from about 1 to 10 seconds and preferably 1 to 5 seconds. Operating 1n accordance with the specified conditions above, heavy fraction is subjected to conversion at a level to provide a desired volume ratio of light cycle gas oil to naphtha products recovered from the process.

Efiiuent from the first and second riser reaction zones, comprising vaporous reaction products and catalyst, discharge into a disengaging space above a fluidized bed of catalyst. In the disengaging space, hydrocarbon vapors separate from used catalyst and the catalyst enters the fluidized bed. From the fluidized bed, used catalyst is withdrawn for stripping and regeneration according to methods well known in the art. Cracked hydrocarbon vapors, substantially free of catalyst, are recovered from the disengaging space and charged to a product fractionatron zone.

In the product fractionation zone, cracked hydrocarbons are separated into at least a gaseous fraction, a naphtha fraction, a light cycle gas oil fraction, and one or more fractions heavier than light cycle gas oil. The

7 naphtha fraction generally boils within the range of about C 430" F. and is suitable for use as gasoline blend stock. The light cycle gas oil boils within the range of about 400650 F;. and has a pour point less than F., preferably less than 0 F. Such light cycle gas oil is useful as furnace oil blend stock.

A small portion of the cracked product comprises hydrocarbons higher boiling than light cycle gas oil. As in conventional fluid catalytic cracking processes, all or a portion of such heavier hydrocarbon may be recycled to a reaction zone for addition conversion. Preferably, however, in the practice of the present invention, such heavier hydrocarbons are yielded as products and all conversion of charge stock is achieved upon a one-pass basis.

EXAMPLE I In order to demonstrate the present invention, a waxy topped crude having the following properties is selected:

Gravity, API 39.0 Characterization factor 12.92

Sulfur, weight percent 0.02 ASTM IBP, F. 525 Pour point, F. '|90 In a first comparative fluid catalytic cracking run the topped crude is cracked under conventional cracking conditions in an elongated riser reaction zone without recycle as follows:

Feed preheat, F. 600 Riser outlet, F. 1015 Residence time, seconds 4 Average riser vapor velocity, feet per second Conversion, vol. percent 71.3

From this conventional cracking reaction, products are recovered and analyzed, as follows:

Octane values of the debutanized naphtha are as follows:

Research, clear 93.7 Research, !+3 cc. TEL 98.3

Light cycle gas oil pour point temperature is +40 R, which is substantially higher than is acceptable for furnace oil blend stock.

EXAMPLE II In a second fluid catalytic cracking run, following the method of the present invention, the topped crude of Example I is separated by fractional distillation into a light fraction and a heavy fraction, having physical properties as follows:

Light Heavy Property fraction fraction Gravity, API 45. 2 37. 0 Sulfur, wt. percent 0. 014 0. 023

Distillation, ASTM IBP.

The light fraction, comprising 25 volume percent of the topped crude and the heavy fraction comprising 75 volume percent are cracked in separate elongated riser reaction zones under cracking conditions as follows:

Light Heavy Cracking condition fraction fraction Feed preheat, F 700 650 Riser outlet, F 1, 055 1, 000 Residence time, seconds- 4 3 Average riser vapor velocity, it. per second 25 35 Conversion, v01. percent..." 90 65 Cracked vapors from both riser reaction zones are combined, separated into product fractions and analyzed as follows:

Octane values of the debutanized naphtha are as follows:

Research, clear 93.8 Research, +3 cc. TEL 98.4

Light cycle gas oil pour point temperature is 5 R, which is well within the acceptable range for a furnace oil blend stock.

By following the method of the present invention, a waxy charge stock containing high pour point components boiling in the furnace oil range, may be converted in a fluidized catalytic cracking process to yield substantial amounts of high octane naphtha and low pour point light cycle oil.

The above description and examples are for specific embodiments of the invention. Other modifications and variations will be obvious to those skilled in the art and are within the scope of the invention. No limitations are intended other than those contained within the appended claims.

We claim:

1. A fluidized catalytic cracking process for converting a waxy hydrocarbon charge stock and producing substantial yields of high octane naphtha and low pour point light cycle oil, which comprise:

(a) separating said charge stock into a light fraction boiling within the range of 400-650" F. and having a pour point greater than about +10 F., and a heavy fraction boiling in the range of 650 F. and higher;

(b) reacting said light fraction, in a first elongated riser reaction zone in the presence of a hot regenerated cracking catalyst at a reaction temperature of from about 950 F. to about 1100 F., and at a conversion of at least about for controlling pour point of light cycle gas oil of step (g) at +10 F. or less;

(c) reacting said heavy fraction, in a second elongated riser reaction zone inthe presence of a hot regenerated cracking catalyst at a reaction temperature of from about 900? F. to about 1000 F. and at a conversion of from about 60% to about 80% for controlling the volume ratio of naphtha to light cycle gas oil recovered in step (g);

-(d) discharging catalyst and hydrocarbon vapor effluent from said first riser and from said second riser ibntg a disengaging space above a fluidized catalyst 1 e (e) separating, in said disengaging space, catalyst, which enters said fluidized bed, from hydrocarbon vapor, which passes overhead from said disengaging space;

(f) withdrawing catalyst from said fluidized bed for stripping and regeneration prior to use in steps (b) and (c) above; and

(g) fractionating, in a product fractionation zone, vapors from said disengaging space into at least a C and lighter fraction, a naphtha fraction, a light cycle gas oil fraction boiling in the range of 400-650 F. and one or more fractions heavier than light cycle gas oil.

2. The process of claim 1 wherein light fraction reaction temperature of step (a) is from about 1000 F. to about 1100 F., wherein light fraction conversion of step (a) is from about 85% to about 95%, and wherein pour point of light cycle gas oil of step (g) is F. or less.

3. The process of claim 1 wherein operating conditions in said first elongated riser reaction zone include, light fraction preheat temperature in the range of about 350- 850 F., catalyst to oil weight ratio of from about 2:1 to 20: l, hydrocarbon residence time within said first riser of about 1-10 seconds, superficial vapor velocity of about 10-60 feet per second, and disengaging space pressure of about -50 p.s.i.g., and wherein such operating conditions are adjusted to obtain light fraction conversion of from about 85 to about 95%.

4. The process of claim 3 wherein operating condition in said second elongated riser reaction zone include, heavy fraction preheat temperature in the range of about 450- 750 F., catalyst to oil weight ratio of from about 2:1 to 20:1, hydrocarbon residence time within said second riser of about l-l0 seconds, superficial vapor velocity of about -60 feet per second, and disengaging space pressure of about 5-50 p.s.i.g., and wherein such operating conditions are adjusted to obtain heavy fraction conversion of from about 60% to about 80% 5. A fluidized catalytic cracking process for conversion of a hydrocarbon charge stock comprising a light fraction having a boiling range of about 550-650 F. and a pour point of at least +20 F., and a heavy fraction boiling above 650 R, which process comprises:

(a) fractionating, in a charge stock fractionation zone, said charge stock into said light fraction and said heavy fraction;

(b) catalytically cracking said light fraction, in a first dilute phase riser cracking zone, at a temperature of from about 950 F. to about 1100 F. andat a conversion of from about 80% to about 100% for maintaining the pour point of light cycle gas oil of step (e) at a +10 F. or less;

(c) catalytically cracking said heavy fraction, in a second dilute phase riser cracking zone, at a temperature of from about 900 F. to about 1000 F. and at a conversion of from about 60% to about 85% for maintaining a selected volume ratio of naphtha to light cycle gas oil in step (e);

(d) discharging vapor and catalyst efiluent from said first riser and said second riser into a disengaging space above a fluidized catalyst bed for separation of catalyst from cracked hydrocarbon vapor; and

(e) fractionating cracked hydrocarbon vapor from said disengaging space, in a product fractionation zone into at least a C and lighter fraction, a debutanized naphtha fraction, a light cycle gas oil frac tion boiling in the range of about 400-650 F. and

one or more fractions higher boiling than light cycle gas oil.

6. The process of claim 5 wherein the light fraction cracking temperature of step (b) is from about 1000 F. to about 1100 F., wherein light fraction conversion of step (b) is from about to about and wherein pour point of light cycle gas oil product is 0 F. or less.

7. A fluidized catalytic cracking process for conversion of a hydrocarbon charge stock comprising a light fraction having a major portion boiling within the range of a light cycle gas oil product of step (f) and having a pour point greater than said light cycle gas oil product, and a heavy fraction having a major portion boiling above the light cycle gas oil product, which process comprises:

(a) fractionating, in a charge stock fractionation zone, said charge stock into said light fraction and said heavy fraction;

(b) catalytically cracking said light fraction in a first dilute phase riser cracking zone, at a temperature of from about 950 -F. to 1100 F. and at a con version of from about 80% to about for controlling the pour point of light cycle gas oil of p (c) catalytically cracking said heavy fraction, in a second dilute phase riser cracking zone, at a temperature of from about 900 F. to about 1000 F. and at a conversion of from about 60% to about 85% for maintaining a selected volume ratio of naphtha to light cycle gas oil in step (f);

(d) separating a first cracked hydrocarbon vapor from the reaction eifluent of said first riser cracking zone;

(e) separating a second cracked hydrocarbon vapor from the reaction efiiuent of said second riser cracking zone; and

(f) fractionating the first hydrocarbon vapor of step (d) and the second hydrocarbon vapor of step (c), in a common fractionation zone, into at least a C and lighter fraction, a debutanized naphtha fraction, a light cycle gas oil fraction, and one or more fractions higher boiling than light cycle gas oil.

8. The process of claim 7 wherein the volume ratio of naphtha to light cycle gas oil is within the range of from about 120.3 to about 1:0.15.

9. The process of claim 8 wherein the pour point of the light cycle gas oil is +10 F. or less.

References Cited OTHER REFERENCES 2,903,414 9/ 1959 Marisic et a1 208-180 3,355,380 11/1967 Luckenbach 208-153 3,433,733' 3/ 1969 Bunn et al. 208- 3,617,496 11/1971 Bryson et al. 208-80 3,630,886 12/1971 Deed et al. 208-96 3,679,576 7/1972 McDonald 208-74 3,689,402 9/1972 Youngblood ct al. 208-93 3,714,024 1/ 1973 Youngblood et al. 208-78 DELBERT E. GANTZ, Primary Examiner G. E. SCHMITKONS, Assistant Examiner US. Cl. X.R. 

